The Book of Household Management by Mrs. Beeton

air, is both singular and instructive. Their bodies are covered with feathers, which are much lighter than coverings of hair, with which quadrupeds are usually clothed. The feathers are so placed as to overlap each other, like the slates or the tiles on the roof of a house. They are also arranged from the fore-part backwards; by which the animals are enabled the more conveniently to cut their way through the air. Their bones are tubular or hollow, and extremely light compared with those of terrestrial animals. This greatly facilitates their rising from the earth, whilst their heads, being comparatively small, their bills shaped like a wedge, their bodies slender, sharp below, and round above,--all these present a union of conditions, favourable, in the last degree, to cutting their way through the aërial element to which they are considered as more peculiarly to belong. With all these conditions, however, birds could not fly without wings. These, therefore, are the instruments by which they have the power of rapid locomotion, and are constructed in such a manner as to be capable of great expansion when struck in a downward direction. If we except, in this action, the slight hollow which takes place on the under-side, they become almost two planes. In order that the downward action may be accomplished to the necessary extent, the muscles which move the wings have been made exceedingly large; so large, indeed, that, in some instances, they have been estimated at not less than a sixth of the weight of the whole body. Therefore, when a bird is on the ground and intends to fly, it takes a leap, and immediately stretching its wings, strikes them out with great force. By this act these are brought into an oblique direction, being turned partly upwards and partly horizontally forwards. That part of the force which has the upward tendency is neutralized by the weight of the bird, whilst the horizontal force serves to carry it forward. The stroke being completed, it moves upon its wings, which, being contracted and having their edges turned upwards, obviate, in a great measure, the resistance of the air. When it is sufficiently elevated, it makes a second stroke downwards, and the impulse of the air again moves it forward. These successive strokes may be regarded as so many leaps taken in the air. When the bird desires to direct its course to the right or the left, it strikes strongly with the opposite wing, which impels it to the proper side. In the motions of the animal, too, the tail takes a prominent part, and acts like the rudder of a ship, except that, instead of sideways, it moves upwards and downwards. If the bird wishes to rise, it raises its tail; and if to fall, it depresses it; and, whilst in a horizontal position, it keeps it steady. There are few who have not observed a pigeon or a crow preserve, for some time, a horizontal flight without any apparent motion of the wings. This is accomplished by the bird having already acquired sufficient velocity, and its wings being parallel to the horizon, meeting with but small resistance from the atmosphere. If it begins to fall, it can easily steer itself upward by means of its tail, till the motion it had acquired is nearly spent, when it must be renewed by a few more strokes of the wings. On alighting, a bird expands its wings and tail fully against the air, as a ship, in tacking round, backs her sails, in order that they may meet with all the resistance possible.